A Shrinkage Compensating Device (SCD) is generally included as a sub-component of a Continuous Tie-down System (CTS). CTS's are a critical element of both single and multi-story plywood shearwall systems, such as those used in multi-unit timber framed residential construction, and are designed to resist the overturning forces associated with those types of shearwalls. A CTS is typically located near each end of a shearwall system, and will generally consist of one or more lengths of rod or all-thread rod interconnected with couplers. One end of the CTS is typically anchored to a fixed foundation element and then attached to each level of the plywood shearwall system with a bearing plate and SCD.
The purpose of the SCD is to compensate for any differential movement that can occur between the CTS and the building (due to such things as moisture related wood shrinkage and building settlement), so as to keep the attachment points between the CTS and the plywood shearwall system tight. By doing this, the vertical uplift related movement of the plywood shearwall is minimized, which in turn also limits the lateral drift or deflection of the shearwall caused by the uplift movement of the CTS.
One type of SCD that can be used in a CTS is referred to as a Ratcheting Type SCD (RT-SCD). A ratcheting type SCD will generally consist of a body element with a frusto-conical bearing surface, a base plate attached to the bottom of the body element, a cap element secured to the top of the body element, a plurality of rod gripping segments located within the body element and configured to engage its frusto-conical bearing surface, and springs positioned between the top of the rod gripping segments and bottom of the cap element so as to provide a constant force that pushes the rod gripping segments down into the frusto-conical bore of the body element. The base plate is provided with holes that allow the SCD to be attached to a timber framing element of the CTS. Examples of RT-SCD's are described in U.S. Pat. No. 6,195,949 (Schuyler), and U.S. Pat. No. 7,752,824 (Brown), and U.S. patent application Ser. No. 11/667,604, Smith.
ICC Evaluation Service (a subsidiary of the International Code Council) has developed an acceptance criteria that is used for the evaluation of SCD's used in CTS's. This document is referred to as AC316 (revised June 2012), and is entitled “Acceptance Criteria for Shrinkage Compensating Devices.” AC316 includes a number of critical performance related provisions for SCD's, including the minimum misalignment tolerance that the SCD must accommodate between the rod and SCD, a limitation to the amount of incremental travel and reseating a RT-SCD may sustain, and limits on the combined displacement between the restraints of a CTS (typically the floor-to-floor attachment points).
Device Average Travel and Seating Increment (DATSI) is the term used in AC316 with regard to the incremental travel and reseating of a RT-SCD. Per Section 1.4.7 of AC316, DATSI: (ΔR) is defined as the average of the movement required to cause incremental motion from a seated position (Δ1) and the opposite motion required to reseat the device after the actuation or ratcheting (Δ2). Thus, ΔR=(Δ1+Δ2)/2. For a typical ratcheting SCD on a standard rod, a minimal theoretical DATSI might include a travel increment (Δ1) equal to 1.40 times the rod thread pitch, and a seating increment (Δ2) equal to 0.40 times the rod thread pitch. Given this, the DATSI a rod with a thread pitch of 10 TPI (Threads Per Inch) works out to be ΔR=(0.100×1.40+0.100×0.40)/2=0.090″. For a rod with a thread pitch of 7 TPI the DATSI works out to be ΔR=(0.143×1.40+0.143×0.40)/2=0.129″.
Per Section 1.4.8 of AC316, the allowable deflection limit of a SCD is defined as ΔA=0.132″−ΔR (based on Allowable Stress Design or ASD). This equation can also be rearranged as ΔA+ΔR=0.132″ (ASD). In addition, per Section 6.0.9 of AC316 there is a new provision that limits the combined displacement from rod elongation, SCD device deformation (ΔA), and DATSI (ΔR) to a maximum of 0.200″ between the restraints of a CTS (typically the floor-to-floor attachment points). Due to this overall displacement limitation it now has becomes very important to keep the DATSI of a SCD to a minimum, so as maximize the displacement allowed the rod, and thus maximizes the load capacity that may be resisted by the rod and associated CTS.
One of the deficiencies with the RT-SCD's that are currently available for use with CTS's is that all of these RT-SCD's currently have rod gripping segments that are configured to engage the standard threads associated with commonly available rod. The problem with this is that these RT-SCD's can have DATSI values that significantly limit the capacity of the CTS per the requirements set forth in AC316, particularly for large diameter rods with reduced thread pitches. In some cases it is possible that the DATSI values for some SCD's will exceed the allowable deflection limit set forth in AC316, and will be restricted from being used in CTS's. As such, it has now become very important that the DATSI values for RT-SCD's be kept to a minimum if the load capacity of the rod and associated CTS is to be maximized. Another problem with the RT-SCD's depicted in the Schuyler and Brown patents, and the Smith patent application, is that due to their configurations, there is little or no tolerance for any misalignment between the rod and RT-SCD. Due to the imperfections inherent in construction, there is a need for RT-SCD's that are capable of providing some misalignment tolerance between the rod and the SCD. As such, AC316 now incorporates minimum misalignment tolerances requirements for SCD's used in CTS's.
Another deficiency with the RT-SCD's that are currently available for use with CTS's is that they do not provide a means that can assist in facilitating the disengagement of the rod gripping segments from the rod. The inclusion of such features would improve the performance and functionality of RT-SCD's, while also reducing their DATSI values.
Another deficiency with the RT-SCD's that are currently available for use with CTS's is that the rod gripping segments sometimes do not all ratchet at the same time as the rod travels through the SCD, particularly when there is a misalignment between the rod and SCD. When this occurs there is the potential that some of the rod gripping segments will not be in bearing contact with the frusto-conical bore of the body element, and could substantially limit the load capacity of the SCD to that provided by the remaining rod gripping segments that have ratcheted, and are in bearing contact with the frusto-conical bore of the body element. As such, there is a need to provide a means that ensures that all of the rod gripping segments of a SCD will ratchet at the same time, or within an acceptable tolerance range. There is also a need to ensure that these RT-SCD's ratchet properly when there is a misalignment between the rod and SCD.
Another deficiency with the RT-SCD's that are currently available for use with CTS's is that the rod gripping segments are typically free to rotate within the body element of the SCD. This condition can present problems if the SCD needs to be uninstalled, as the only way to remove one of these SCD's is to spin the SCD back up the rod. If the rod gripping segments get jammed or locked onto the threads of the rod, then the body will only spin around the rod gripping segments, making it very difficult, if not impossible to remove the SCD without cutting the rod. To avoid this situation there needs to be provided a means to positively lock the body element of an SCD to the rod gripping segments, and thus allow the SCD to be spun back up the rod as required.
As spinning an RT-SCD back up a rod can take time, it would also be very beneficial to provide the means to disengage the rod gripping segments of an RT-SCD from the rod so as to allow the SCD to be quickly uninstalled by just sliding the SCD back up the rod. This is not provided in any of the RT-SCD's currently available for use with CTS's.
The present invention addresses the above mentioned deficiencies in the prior art RT-SCD's by providing improvements to these SCD's that allow for increased RT-SCD performance, functionality, dependability, and higher allowable load capacities for the overall CTS.